1. Field of the Invention
The present invention relates to a power circuit including a high voltage transformer.
2. Related Background Art
In the prior art, as a power system for an electrophotographic image forming system such as a copying machine, a system which activate a low voltage output for operating a system logic circuit, a motor or a solenoid and a high voltage output for a charger has been proposed and put into practice.
As an example of a prior art power system of this type, a block configuration of a hybrid power circuit mounted on a copying machine is shown in FIG. 18. In FIG. 18, numeral 1 denotes a commercial AC power line, numeral 2 denotes a rectifying bridge, numeral 3 denotes a first filtering capacitor, numeral 4 denotes an input winding drive switching element, numeral 5 denotes a resonance capacitor which functions to generate a voltage resonance in cooperation with an inductance component of a hybrid transformer 20 to be described later, numeral 6 denotes a clamping diode, numeral 7 denotes a current transformer for monitoring currents flowing in the input winding drive switching element 4, the resonance capacitor 5 and the clamping diode 6, and numeral 8 denotes a drive transformer for temporarily transmitting a switching element drive signal from a main control unit 9 in a secondary circuit. The main control unit 9 controls an on-operation time of a primary winding 21 in accordance with a detection result of a rectified output voltage of a low voltage flyback winding 22. The main control unit 9 comprises a drive signal output circuit 9a for the switching element 4, primary circuit drive current detection signal input circuit 9b and a 24 VR filtered voltage detection signal input circuit 9c.
Further, in FIG. 18, numeral 10 denotes a diode for rectifying an output of the low voltage flyback winding 22 and numeral 11 denotes a filtering capacitor. The output of the winding 22 in the present example is controlled to 24 volts and it is referred to as 24 VR. Numeral 112 denotes a voltage detection resistor for detecting the 24 VR voltage, numeral 13 denotes a diode for rectifying an output of a low voltage forward winding 23, and numeral 14 denotes a switching element which turns on and off such that the filtered output of the low voltage forward winding 23 is maintained at a predetermined voltage. The output of the winding 23 in the present example is controlled to 24 volts and it is referred to as 24 VU. Numeral 15 denotes a flywheel diode, numeral 16 denotes a rectifying inductance, numeral 17 denotes a filtering capacitor, numeral 18 denotes an output detection resistor for detecting the 24 VU output, numeral 19 denotes a sub-control unit which controls a conduction angle of the switching element 14 such that the 24 VU output is maintained at a predetermined voltage. The sub-control unit comprises a drive signal output circuit 19a for the VU switching element 14, a 24 VU filtered output detection signal input circuit 19b and a drive signal input circuit 19c for the switching element 4.
Further, in FIG. 18, numeral 20 denotes a hybrid transformer which comprises a primary winding 21, a low voltage flyback winding 22, a low voltage forward winding 23 and high voltage winding 24 and 25. Numeral 26 denotes a diode for rectifying the output of the high voltage winding 24 to a negative voltage, numeral 27 denotes a filtering capacitor, numeral 28 denotes a diode for rectifying the output of the high voltage winding 25 to a positive voltage and numeral 29 denotes a filtering capacitor.
An operation of the power circuit of the above configuration is now explained.
A current from the commercial AC power line 1 is rectified and filtered by the rectifying bridge 2 and the filtering capacitor 3 and applied to one end of the primary winding 21 of the hybrid transformer 20. During the on-time of the switching element 4, a current flowing in the switching element 4 linearly increases with a gradient which is inversely proportional to an input inductance of the hybrid transformer 20. During the on-time of the switching element 14 for the 24 VU winding output, a winding current thereof is superimposed on the primary circuit switching element 4 inversely proportionally to a turn ratio.
When the primary circuit switching element 4 is turned off, an LC resonance by the primary circuit inductance and the resonance capacitor 5 is generated in accordance with a current energy of that time. A voltage of the resonance capacitor 5 is substantially sinusoidal wave. A negative side is limited to a diode forward voltage by the clamping diode 6. The main control unit 9 monitors the status by the current transformer 7 and when it detects the flow-in of the current into the negative side, it generates the on-signal again to the switching element 4. The low voltage flyback winding 22 of the hybrid transformer 20 is rectified and filtered at a phase of a resonance peak and this voltage is monitored by the main control unit 9. When the output of the winding 22 is lower than 24 volts, the primary control circuit 9 in the secondary circuit increase the on-period of the switching element 4, and when it is higher, it decreases the on-period.
In this manner, the switching at 0 volt is always attained for the switching element 4 and the rectified voltage of the low voltage flyback winding 22 is controlled to 24 volts. On the other hand, other output windings output the output voltage in accordance with a turn ratio of the hybrid transformer 20.
In this manner, the LC flyback voltage resonance is generated in the input circuit of the hybrid transformer 20 and the peak current value of the output of the low voltage flyback winding 22 is controlled to the predetermined voltage in the output circuit so that the flyback circuit high voltage outputs having the peak value thereof controlled to the constant voltage are generated in the high voltage output windings 24 and 25 of the hybrid transformer 20.
In general, the main control unit 9 is added with a so-called soft start function to gradually raise in time a 24 VR control target voltage so that a predetermined time is spent for the rise to a 24 VR final target in order to reduce transient current and burdens of related components.
The 24 VU winding output is poled to rectify the so-called forward circuit output and in general, it may be larger than the output voltage of the 24 VR, but since the forward circuit voltages of the respective winding of the hybrid transformer 20 are substantially proportional to the filtered voltage of the AC input voltage, individual voltage controls may be required in the secondary circuit depending on the precision required to the output voltages. The 24 VU voltage control unit (sub-control unit) 19 monitors the 24 VU current and voltage, and changes the conduction rate of the switching element 14 to control the output voltage to the predetermined voltage.
Main operation waveforms in a steady state is shown in FIGS. 19A to 19D. FIGS. 19A to 19D show steady state operations when a 24 VU external load is in an on-state.
FIG. 19A shows an operation waveform of an MPWM, FIG. 19B shows an operation waveform of a collector voltage of the switching element 4, FIG. 19C shows an operation waveform of a collector current of the switching element 4 and FIG. 19D shows an operation waveform of the 24 VUSW (switching element 14).
In an electrostatic electrophotographic image forming system of the prior art, several types of high voltage circuits which output several hundreds voltages to several tens kilovolts are provided for the purpose of the charging to a photo-conductor, the development, the transfer to a transfer sheet and the separation of the transfer sheet from the photo-conductor.
A predetermined on/off signal, an output control signal and an output current control signals are received in accordance with the purpose to control the output. Recently, as a finer optimization trend of an image forming condition, a demand to switch the output voltage by an electrical impedance of the high voltage output load of the charging unit or the transfer unit to a more optimum value has been arisen. Namely, it intends to positively respond to a change of the electrical impedance by the change of environment and aging.
In order to achieve the above object, a high voltage power circuit which outputs a high voltage at a predetermined constant current value when an image forming operation is not conducted, detects the output voltage at that time and calculates a load resistance, and determines the output voltage value or the output current value in accordance with the result thereof has been put into practice.
FIG. 23 shows a block configuration of a prior art high voltage power circuit of this type. In FIG. 23, numeral 30 denotes a high voltage transformer which, in the present example, comprises an output winding 31, an input winding 32, a detection winding 33, a high voltage use rectifying diode 34 and a bleeder resistor 35 connected to the output winding 31, numeral 36 denotes a follower transistor for transmitting a voltage to an input circuit of the high voltage transformer 30 in accordance with a result of comparison by the control circuit, numeral 37 a filtering capacitor for supplying a stable DC voltage to the input winding of the high voltage transformer 30, numeral 38 denotes an oscillator for generating a high frequency signal to drive the high voltage transformer 30, numeral 39 denotes a switching transistor, numeral 40 denotes a resonance capacitor for generating an LC flyback resonance with an input inductance of the high voltage transformer 20 and numeral 41 denotes a clamping diode for clamping the intrusion of the resonance waveform into a negative side.
Further, in FIG. 23, numeral 42 denotes an output (series) resistor for detecting a current to the load which is selected to a high resistance in the order of several M .OMEGA. to several tens M .OMEGA., numeral 43 denotes a load current detection circuit for detecting the current to the load and comprises resistors 44 and 45 and a capacitor 46, numeral 47 denotes a buffer circuit for connecting the voltage of the load current detection circuit 43 to a succeeding stage with a high input resistance, numeral 48 denotes an operational amplifier for comparing the detected current value and a target set value, numeral 49 denotes a control target set voltage, numeral 50 denotes a filter circuit to render the output voltage of the filter circuit 50 to exhibit a desired frequency characteristic, numeral 51 denotes a diode for transmitting the output voltage of the filter circuit 50 to the input circuit of the high voltage transformer 20 and numeral 52 denotes a constant voltage control block which functions as a constant voltage control loop when a voltage larger than the output of the filter circuit 50 is outputted. The constant voltage control block 52 comprises transistors 53 and 54, a diode 55, resistors 56, 57, 58, 59, 60 and 61 and a capacitor 62.
The high voltage circuit unit 63 is configured in the above manner.
Further, in FIG. 23, numeral 63 denotes a load connected to the output of the high voltage circuit unit 63 and it assumes a parallel circuit of the resistor 65 and the capacitor 66 as an electrical characteristic, numeral 67 denotes a power controller which has a function to control the overall operation of the high voltage power circuit and the operation thereof is controlled by an external control unit which controls the overall operation of the high voltage power circuit, and numeral 68 denotes a rectifying/filtering circuit which comprises a diode 69, resistors 70, 71 and 72 and a capacitor 73.
An operation of the above high voltage circuit unit 63 is now explained.
First, the external control unit (not shown) (which is a controller for the overall high voltage power circuit which remotely controls the operation of the high voltage circuit unit 63) receives a load resistance measurement value start mode signal (not shown). In response thereto, a high voltage output enable signal is sent to the oscillator 33 in the off-state of the output voltage of the constant voltage control block 52. Thus, the oscillator 38 starts the oscillation at predetermined frequency and duty to switch the switching transistor 39. During the on-period of the switching transistor 39, a predetermined energy is stored in the high voltage transformer 30 in accordance with the input voltage, the on-time and the input impedance of the high voltage transformer 30. On the other hand, during the off-period of the switching transistor 39, the energy causes the LC resonance by the input impedance of the high voltage transformer 30 and the resonance capacitor 40 and a so-called flyback voltage is generated at the collector of the switching transistor 39. This voltage is transmitted to other windings in accordance with the turn ratio so that a higher voltage is generated in an output winding of a high turn ratio. It is supplied to the external load 53 through the rectifying diode 36. In general, a substantially peak voltage V.sub.o of the output winding is held by a component of the capacitor 66 of the external load 64 and a DC high voltage output is supplied. A external load current I.sub.d at this time is determined by a formula (1): EQU I.sub.d =V.sub.o /(R1+R2+R3) (1)
where R1 is a resistance of the resistor 65, R" is a resistance of the resistor 42 and R3 is a total impedance of the load current detection circuit 43. PA1 where R1 is a resistance of the resistor 91, R2 is a resistance of the resistor 82 and R3 is a total impedance of the load current detection circuit 83.
The value of the current detection point Vdet is dropped by the external load current. When the value of the current detection point Vdet is larger than the value of the control target set voltage 49, the output of the operational amplifier 48 rises and the input voltage of the high voltage transformer 30 rises. Thus, the output voltage of the high voltage transformer rises and the load current increases to operate to reduce the value of the current detection point Vdet. Inversely, when the value of the current detection point Vdet is lower than the control target set voltage 49, the output voltage of the operational amplifier 48 drops and the input voltage of the high voltage transformer 30 drops.
In this manner, the output of the high voltage transformer 30 is regulated such that the load current is equal to the set value. Under this condition, the output of the output voltage detection winding 33 is inputted to an A/D port of the power controller 67 through the rectifying/filtering circuit 68 to calculate the high voltage output voltage.
In this manner, the load current and voltage are determined and the load impedance is determined, and an optimum high voltage output value is determined by the calculation in accordance with the result.
Thereafter, in response to the reception of an image area high voltage output on signal (not shown) from the external control unit, the output voltage of the constant voltage output control block 52 is set such that the previously determined optimum output voltage is outputted. In the present example, since the constant current control loop is always closed by the diode 51, only the output voltage which is larger than the output voltage in the constant current output mode may be set. The output voltage is feedback controlled by serially monitoring the voltage of the detection winding 33 so that the constant voltage control is conducted.
For a high voltage output circuit which controls the load current to a predetermined value, load current detection means is provided in the power circuit and the input voltage to the transformer is controlled in accordance with the output value of the detection means so that the load current is controlled to the desired constant value.
FIG. 24 shows a block configuration of a prior art power circuit which conducts such control. In FIG. 24, numeral 74 denotes a high voltage transformer which, in the present example, comprises an output winding 75a, an input winding 75b and a high voltage use rectifying diode connected to the output winding 75a, numeral 77 denotes a filtering capacitor for supplying a DC voltage to the input winding 75b, numeral 78 denotes an oscillator (OSC) for generating a high frequency signal to drive the high voltage transformer 74, numeral 79 denotes a switching transistor, numeral 80 denotes a resonance capacitor for causing the LC flyback resonance with the input impedance of the high voltage transformer 74, numeral 81 denotes a clamping diode for limiting the intrusion of the resonance waveform into the negative side, numeral 82 denotes a resistor for detecting the current to the load which is selected as an output (series) resistor having a high resistance of several M .OMEGA. to several tens M .OMEGA., numeral 83 denotes a load current detection circuit for detecting the current to the load, numeral 84 denotes a buffer circuit for connecting the voltage of the load current detection circuit 83 to a succeeding stage with a high input resistance, numeral 85 denotes an operational amplifier for comparing the detected current value and the control target set voltage value, numeral 86 denotes a control target set voltage, numeral 87 a filter circuit to render the output voltage of the control loop to exhibit the desired frequency characteristic and numeral 88 denotes an emitter follower transistor for transmitting the output voltage of the filter circuit 87 to the input circuit of the high voltage transformer 74.
Those components constitute the high voltage circuit unit 89.
Further, in FIG. 24, numeral 90 denotes a load connected to the output circuit of the high voltage circuit unit 89, which in the case of electrography, corresponds to a charger and assumes a parallel circuit of a resistor 91 and a capacitor 92 as an electrical characteristic, numeral 93 denotes an external control unit which has a control function to control an overall operation of the power circuit and numeral 94 denotes a signal line for transmitting a setting value of a load current from the external control unit 93 to the high voltage circuit unit 89 for setting a voltage which is a control target voltage value of the operational amplifier 85. Various transmission systems may be used although they are not specifically referred here. Numeral 95 denotes a signal line for setting the output on/off from the external control unit 93 to the high voltage circuit unit 89. In the present example, an oscillation start/stop signal is transmitted to the oscillator 78.
An operation in a constant current mode in the high voltage circuit unit 89 of the above configuration is now explained.
First, a current setting value is set from the external control unit 93 to the high voltage circuit unit 89. In the present example, the load current detection circuit 83 is constructed to exhibit a current-voltage characteristics as shown in FIG. 25. Namely, when the load current is 0, the detected voltage is 4 volts, and as the load current increases, the detected voltage linearly decreases. Accordingly, when the current setting is conducted, a value smaller than 4 volts is set as the control target voltage value of the operational amplifier 85.
Then, the output on/off signal is turned on. Thus, the oscillator 78 starts the oscillation at the predetermined frequency and duty to switch the switching transistor 79. During the on-period of the switching transistor 79, a predetermined energy is stored in the high voltage transformer 74 in accordance with the input voltage, the on-time and the input inductance of the high voltage transformer 74. On the other hand, during the off-period of the switching transistor 79, the energy causes the LC resonance by the input inductance of the high voltage transformer 74 and the resonance capacitor 80 so that a so-called flyback voltage is generated at the collector of the switching transistor 88. This voltage is transmitted to other windings in accordance with the turn ratio so that a higher voltage is generated in the output winding 75a of a large number of turns. It is supplied to the external load 90 through the rectifying diode 76. In general, a substantially peak value V.sub.o of the output winding is held by the component of the capacitor 92 of the external load 90 and a DC high voltage output is supplied to the external load 90. The external load current I.sub.d at this time is determined by: EQU I.sub.d =V.sub.o /(R1+R2+R3) (1)
The value of the current detection point Vdet is dropped by the external load current. When the value of the current detection point Vdet is larger than the control target set value 86, the output voltage of the operational amplifier 85 rises and the input voltage of the high voltage transformer 74 rises. Thus, the output voltage of the high voltage transformer 74 rises and the load current increases to reduce the value of the current detection point Vdet. Inversely, when the value of the current detection point Vdet is lower than the value of the control target set voltage 86, the output voltage of the operational amplifier 85 drops and the input voltage of the high voltage transformer 74 drops.
In this manner, the output voltage of the high voltage transformer 74 is regulated such that the load current is equal to the set value.
In the above example, the constant high voltage output current control is attained by increasing and decreasing the magnitude of the input voltage of the high voltage transformer 74. In other example, the duration of the on-time in the switching drive of the high voltage transformer 74 is increased and decreased to attain the constant high voltage output current control.
However, the circuit shown in FIG. 18 raises the following problems.
(1) Since the output current of the secondary circuit forward winding is superimposed on the drive current of the primary circuit switching element 4 inversely proportionally to the turn ratio, a very large drive current is superimposed on the primary circuit switching element 4 during the rise period in which the voltage of the 24 VU filtering capacitor is small and the charging current is large so that a burden is imposed to the related components.
(2) Since the above state occurs before the primary circuit LC voltage resonance which is the main control drive source shifts to the oscillation state by the stable 0 volt switching, the transition period to shift to the stable resonance state is extended and the burden is imposed to the related components for a longer period.
This manner is explained with reference to FIGS. 20A to 20C and FIGS. 21A to 21D.
FIGS. 20A to 20C show the outputs in the circuit of FIG. 18, that is, the 24 VR and 24 VU voltage rise waveforms. FIG. 20A shows the 24 VR voltage rise waveform, FIG. 20B shows the 24 VU voltage rise waveform and FIG. 20C shows the period.
The 24 VR is controlled to reach the final target value with the predetermined time by the soft start control function by the gentle extension of the on-time of the main control pulse (24 VU switching element control signal) 19a or the gentle rise of the control reference value by the main control unit. On the other hand, for the 24 VU, because of the nature of the source winding in the forward circuit, the output having the substantially final value is supplied from the winding even during the soft start so that the final target value is rapidly reached.
FIGS. 21A to 21D show waveforms of the 24 VU control signal in the periods a, b and c of FIG. 20C, a waveform of the main control signal (24 VR control signal), a waveform of the drive current of the primary circuit switching element 4 and a waveform of the collector voltage. FIG. 21A shows the waveform of the 24 VU control signal, FIG. 21B shows the waveform of the main control signal (24 VR control signal), FIG. 21C shows the waveform of the primary circuit switching element 4 and FIG. 21D shows the waveform of the collector voltage.
In the period a, the initial manner of the 24 VR rise is shown. The main control signal is started by the PWM signal having a narrow on-period by the soft start function. On the other hand, since the 24 VU is much lower than the target value, the 24 VU control signal is totally on during the forward circuit on-period. (It is the PWM signal substantially identical to the main control signal). At this time, the 24 VU forward circuit winding output is already at the final value as described above. The 24 VU forward circuit winding output is a turn ratio equivalent (V.sub.24Uout) of the DC input voltage of the primary circuit, and since the charging voltage (24 VU(t)) of the secondary circuit output capacitor is low, most of it is applied to the filtering inductance (L.sub.24U) so that a large winding output current (and a large time gradient .alpha.) are generated. The secondary circuit current becomes the drive current of the primary circuit switching element 4 in accordance with the turn ratio.
As the time transit to the periods b and c, the voltage of the 24 VU output capacitor rises, the secondary circuit winding output current naturally decreases and finally the winding output current is substantially 0 if the 24 VU external load is not on. At this time, the primary circuit current is the drive circuit for merely storing the 24 VR flyback energy. (The load current of the high voltage winding may be neglected because it is generally very small.) The gradient of the primary circuit current increase at this time is substantially determined by the primary inductance of the high voltage transformer 20 and the DC input voltage. In the period b, it is in an intermediate state.
In the period a, the primary circuit drive current may be equal to or larger than a steady state (24 VU rated external load current flows) maximum value depending on a circuit design condition. In such a case, a protection circuit configuration (not shown) to protect against an overcurrent by detecting the drive current of the primary circuit switching element 4 may not be attained or a protection level should be set to much lower than that expected from the rated load.
The above has been discussed primarily on the problem (1). The problem (2) is now discussed with reference to FIGS. 22A and 22B.
FIGS. 22A and 22B show waveforms of the drive current of the primary circuit switching element 4 and the collector voltage in the three start periods a, b and c. FIG. 22A shows the drive current waveform, FIG. 22B shows the collector voltage waveform, and a shows the period immediately after the start of the output of the main control pulse. The main control pulse starts from a state in which the DC voltage applied to the other end of the primary winding is fully charged in the resonance capacitor 5. The pulse width is controlled to start from the narrowest width (soft start) but since the switching elements 4 shorts the charging load of the resonance capacitor 5, a very large current flows during this period as shown. During this period, a sufficiently large current to cause the LC resonance naturally does not flow and the flyback waveform cannot jump back to 0 volt.
Thereafter, before the voltage resonance waveform by the perfect 0 volt switching as in the period c is attained, there is a transient period as the period b in which flyback waveform gradually grows but the flyback energy is yet low and the normal 0 volt switching is not attained. When the 24 VU charging current is superimposed during this period, the primary circuit current waveform as shown at the top of FIGS. 22A and 22B appears and the current which shorts the remaining voltage of the resonance capacitor 5 by the switching element 4 and the superimposing current from the secondary circuit are combined so that a large current load is imposed to the switching element 4.
When the main control has a control algorithm to detect the moment of switching of the primary circuit drive current from positive to negative to start the next on-operation, the intrusion of the flyback resonance waveform to 0 volt is further impeded during the transient period in which the 24 VU charging current is superimposed on the primary circuit in the positive direction and the shift to voltage resonance control state by the normal 0 volt switching is further delayed.
In the circuit shown in FIG. 23, during the period in which the output voltage is small for the constant current output I and the current for charging the capacitor component occupies a large portion of the total load current for the current flowing in accordance with the load resistance, the output current is given by EQU (i.times.t)/c (2)
and it is a waveform which linearly increases in time with a gradient proportional to the constant current value. When a parallel resistance component r is considered together, the time change for the final output is represented by {1-exp (-t/cr)} where cr is a time constant. In this case, a long time may be needed to reach the final output value depending on the resistance component and the capacitance component of the output load. For example, when the load capacitance is 200 pF and the load resistance is 500 M.OMEGA., the time constant or is approximately 100 ms and a time longer than that is naturally required to attain the output closer to the final value.
It has been described above that the measurement of the load impedance is conducted while the image forming operation is not conducted. For example, when the load impedance is measured before the image forming operation, the load impedance measurement period is directly added to the so-called fast copy time depending on the pre-processing sequence of the copying operation and the extension of the fast copy time cannot be neglected and faster measurement method is desired.
Further, in the circuit shown in FIG. 24, when the resistance of the external load circuit becomes very high resistive by any cause, for example, when a trouble occurs in a supply circuit from the output of the high voltage power unit to the external load or a connection cable is disconnected by some cause, the load current does not flow and the constant current control loop increases the input voltage of the high voltage transformer up to the maximum capacity of the circuit unit or increases the on-time so that the output voltage largely exceeds the maximum output which is required for the normal load.